There continues to be ever increasing demand for distributed high speed access to computer networks such as the Internet and private networks. Competition is fierce among various schemes which rely upon wires for physical layer connectivity, such as T1 carrier, Digital Subscriber Line (xDSL), cable modem, fiber optic distributed data interface (FDDI), and the like. However, it is readily apparent that wireless access systems continue to hold the promise of reducing network buildout costs, especially in areas where telephone, cable and/or fiber optic lines are not yet installed. Wireless systems almost always promise the most rapid and flexible deployment of access services and a quicker return on investment.
Certain radio frequency bands have been allocated in the United States and in other countries to provide so-called Local Multipoint Distribution Service (LMDS). LMDS uses super high frequency microwave signals in the 28 or 40 gigahertz (GHz) band to send and receive broadband data signals within a given area, or cell, approximately up to six miles in diameter. On the surface, LMDS systems work in a manner analogous to that of narrow band cellular telephone systems. In the typical LMDS system, a hub transceiver services several different subscriber locations. The antenna at the hub has a wide viewing angle to allow access by multiple subscribers that use individual narrowly focused subscriber antennas. A high speed data communication service is provided by deploying appropriate modem equipment at both the hub and subscriber locations. Depending upon the particular modems used, the services provided to each subscriber can be, for example, a point-to-point dedicated service.
This type of service can compete directly with wired services available from telephone companies and cable company networks. However, the designers of LMDS systems are faced with several challenges at the present time. Because such systems send very high frequency radio signals over short line-of-sight distances, cell layout has proven to be a complex issue. Some factors that must be considered in cell site design are line of sight, analog versus digital modulation, overlapping cells versus single is transmitter cells, transmit and receive antenna height, foliage density, and expected rainfall. The configuration of antennas and transceivers at a hub site determines the specific coverage of the different sectors within a cell. Antennas with wide viewing angles result in fewer sectors at each cell site. Narrow sectors can be established, but narrower sectors require more hub equipment to cover the same field of view. Also, narrow sectors using the same polarization increase the amount of interference from one hub to the other. Wireless communication system designers can overcome this limitation by using polarization diversity at a cell site. In one approach, narrow sectors using orthogonal polarizations (i.e., the signals radiated from two hubs are 90 degrees to one another) are interleaved to reduce the interference. This polarization diversity can be achieved using orthogonally polarized antennas with very low cross-polarization levels. However, the design of antennas with low cross-polarization levels throughout the sector remains a challenge.
Another challenge is in the electronics technology needed to implement the service. For example, transmitter amplifiers for such high frequency systems require sophisticated semiconductor technology such as using monolithic millimeter-wave integrated circuits (MMICs) based on gallium arsenide technologies. These MMICs generate considerable heat in the transceiver unit and the heat needs to be dissipated by careful design of the heat sink of the transceiver. Furthermore, transceiver systems must provide precise control over signal levels in order to affect the maximum possible link margin at the receiver.
One overriding concern with LMDS services is that they are fixed services and therefore have certain properties that are dramatically different than for mobile services. One difference in particular is that LMDS service is completely line of sight, meaning that a clear path for signal propagation between the hub and subscriber is an absolute requirement. Locations without direct line of sight access typically require auxiliary reflectors and/or amplifiers, if they can be made to work at all.
Another consideration in an LMDS system is that connection is expected to be full duplex, in the sense that the transmitter is expected to operate at the same time as the receiver, with minimal interference being generated between them. Thus, broadband communication systems such as LMDS require a highly directional (i.e., narrowly focused) antenna that has very low cross-polarization levels throughout the viewing area. Also, since these transceiver equipments are used for subscriber units, these need to be small, compact and should fit in with the decor of the subscriber dwellings. An additional advantage would be provided if some type of heat dissipation capability was also provisioned for the unit.
Certain compact microwave and millimeter-wave radars operating at extremely high frequencies have been developed using a folded folding optics design. Such a design uses an external lens for focusing electromagnetic radiation to define an antenna axis. A separate transreflector placed in a plane orthogonal to the axis of the lens and a separate twist reflector assembly is also placed in the same plane. Such assemblies typically require fabrication of multiple individual components. See, for example, the antennas described in U.S. Pat. No. 5,455,589 issued to Huguenin, G. R. and Moore, E. L. on Oct. 3, 1995 and assigned to the Assignee of the present application, as well as U.S. Pat. No. 5,680,139 issued on Oct. 21, 1997 to the same inventors, and also to the same Assignee.